Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treating, melting, and annealing. Such torches include an electrode which supports an electric arc that extends from the electrode to a workpiece. A plasma gas such as an oxidizing gas is typically directed to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. In some types of torches, a second shielding gas is used to surround the jet of plasma gas and the arc for controlling the work operation. In other types of torches, a swirling jet of water is used to surround the jet of plasma gas and the arc and impinge on the workpiece for controlling the work operation.
In a variety of circumstances, it is desired to cut metal workpieces along cutting paths that are at least partly arcuate in shape such that the torch has a nonzero angular velocity during at least portions of the cutting operation. The advance rate of the torch in surface feet per minute is generally a function primarily of the type and thickness of the material being cut and the current density of the torch expressed in amps of arc current per square inch of nozzle area. Thus, in existing plasma arc cutting methods, the advance rate of the torch typically is selected independently of the shape or contour of the cutting path along which the torch is moved. Accordingly, when the torch is moving along an arcuate path, the angular rate of movement of the torch increases in an inversely proportional manner to the radius of curvature of the cutting path.
A phenomenon which has been noted in cutting small holes (e.g., hole diameters of about 1 inch or less) with a plasma arc torch is that the increased angular rate of the torch results in the arc not following the desired noncircular or circular path, but rather "whipping" around. Although not wishing to be bound by theory, it is thought that centrifugal effects become more and more significant as the angular velocity of the torch increases, such that the centrifugal effects are great enough to influence the movement of the arc, perhaps because the plasma gas flow does not follow the torch as accurately as it does at lower angular velocities. The result of this arc whipping is that the workpiece is cut along a path that does not conform to the desired cutting path. Problems of nonconformance are especially likely at the end of a hole cut where the finishing end of the cutting path joins the starting end of the cutting path. However, nonconformance caused by arc whipping can result whenever the torch is moved along a nonlinear path during a cutting operation.